Nicene and Post-Nicene Fathers, Vol. V:
A Treatise on the Merits and Forgiveness...: Chapter 23

p. 54

Chapter 23 [XXI.]—Why God Prescribes What He Knows Cannot Be Observed.

We must not deny that God commands that we ought to be so perfect in doing righteousness, as to have no sin at all. Now that cannot be sin, whatever it may be, unless God has enjoined that it shall not be. Why then, they ask, does He command what He knows no man living will perform? In this manner it may also be asked, Why He commanded the first human beings, who were only two, what He knew they would not obey? For it must not be pretended that He issued that command, that some of us might obey it, if they did not; for, that they should not partake of the fruit of the particular tree, God commanded them, and none besides. Because, as He knew what amount of righteousness they would fail to perform, so did He also know what righteous measures He meant Himself to adopt concerning them. In the same way, then, He orders all men to commit no sin, although He knows beforehand that no man will fulfil the command; in order that He may, in the case of all who impiously and condemnably despise His precepts, Himself do what is just in their condemnation; and, in the case of all who while obediently and piously pressing on in his precepts, though failing to observe to the utmost all things which He has enjoined, do yet forgive others as they wish to to be forgiven themselves, Himself do what is good in their cleansing. For how can forgiveness be bestowed by God’s mercy on the forgiving, when there is no sin? or how prohibition fail to be given by the justice of God, when there is sin?